1. Field of the Invention
The present invention relates to a process for the cleavage of alkyl tert-alkyl ethers by reactive distillation in the presence of acid catalysts to give the corresponding olefins and alkanols.
2. Background of the Invention
The cleavage of ethers, in particular alkyl tert-alkyl ethers, into alkanols and olefins, is a known reaction and can be used for the preparation of pure olefins. Thus, for example, isobutene is produced in technical-grade purity by dehydrogenation of C4 mixtures. These C4 mixtures comprise, apart from traces of C3 and C5 compounds, isobutene, 1-butene and 2-butene. Separation of this mixture by simple distillation to isolate pure isobutene is uneconomical because of the very small boiling point difference or separation factor for 1-butene and isobutene.
Pure isobutene is therefore usually prepared by cleavage of methyl tert-butyl ether (MTBE) back into isobutene and methanol.
The acid-catalyzed cleavage of ethers such as MTBE to prepare pure olefins such as isobutene is a process known per se. A distinction is made here between two different process variants. First, the cleavage can be conducted in the liquid phase over acid ion exchange resins as described, for example, in DE 3 509 292 A1 or DE 3 610 704 A1 or over acidic aluminum oxides as disclosed, for example, in DD 240 739 A1. In the latter case, the reaction conditions (167xc2x0 C. and 1 bar or 297xc2x0 C. and 10 bar) are selected so that the MTBE cleavage occurs in the gas/liquid region or in the pure gas phase.
Secondly, the cleavage reaction can be conducted in the gas/liquid phase in a type of combined reaction distillation column over acid catalysts, as disclosed in EP 0 302 336 A1 or DE 43 22 712. EP 0 302 336 A1 describes the elimination of methanol from MTBE over an acid ion exchange resin which is positioned in the bottom of the column. The cleavage of the ether here takes place in the bottom of the column, i.e. the catalyst is continually surrounded by a mixture of ether, olefin and alcohol. This is a disadvantage for the preparation of isobutene, since, first, it does not ensure that the isobutene which oligomerizes readily under acid conditions is removed quickly and, second, the acid centers of the catalyst are occupied by methanol. A different route is taken in DE 43 22 712. In this document, the tertiary ether is fed into a reaction distillation column above the reaction zone, and the rectification section of the column serves to purify the isobutene, while in the stripping section of the column, methanol is separated from the MTBE/methanol azeotrope. The azeotrope goes back into the reaction zone. Sulfated titanium dioxide extrudate is used as the acid catalyst.
In both procedures, catalyst poisons present in the feed, for example metal ions, can deactivate the Bronsted acid catalyst. In addition, the introduction of an MTBE/methanol mixture in this arrangement would decrease the rate of the MTBE cleavage reaction and thus reduce conversion. Methanol inhibits the actual cleavage reaction because it occupies the acid centers of the catalyst.
In the case of cleavage processes which are conducted in the pure liquid phase, it has to be noted that high MTBE conversions cannot be achieved in principle. This is because the cleavage reaction is a typical equilibrium reaction. Thus, for example, a liquid phase reaction at equilibrium at 100xc2x0 C. and the corresponding total pressure has the following composition:
mol. fraction of isobutene=xcx9c15 mol. %
mol. fraction of MTBE=xcx9c70 mol. %
mol. fraction of methanol=xcx9c15 mol. %
Another problem of this process is the isobutene which is dissolved in the homogeneous liquid phase, which can undergo subsequent reactions. The most important reactions of this type are acid-catalyzed dimerization and oligomerization. For this reason, undesired C8 and C12 components are also found in addition to the desired isobutene product. The undesired C8 molecules are 2,4,4, -trimethyl-1-pentene and 2,4,4, -trimethyl-2-pentene. Furthermore, because of the sometimes high reaction temperature, a subsequent reaction in which two methanol molecules react to form water and dimethyl ether. Since this reaction results in a considerable loss of methanol, fresh methanol has to be fed into the reaction, especially if the cleavage is integrated in a circuit with an MTBE synthesis.
In the process variant in which the cleavage reaction is conducted in the pure gas phase, the problems of dimerization or oligomerization of the isobutene formed to undesirable downstream products likewise occur. Dilution of the gaseous starting material stream with inert gas can reduce these reactions, but not eliminate them entirely. Dilution of the starting material stream at the same time reduces the efficiency of the production plant.
The reactions conducted in the gas phase or at high temperatures in the processes described have the disadvantage that high-boiling cracking products are formed during the cleavage process and deposit on the catalyst, thus deactivating it. Deactivated catalysts and/or high temperatures favor the formation of by-products and reduce the selectivity of the reaction. Particularly, the isobutenes obtained by ether cleavage tend to undergo undesirable thermal polymerization. If the cleavage reaction is conducted at lower temperatures, low conversions frequently result.
Accordingly, one object of the present invention is to provide a process for the cleavage of alkyl tert-alkyl ethers which achieves a high ether conversion together with low downstream product formation and low catalyst deactivation.
Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by a process for the cleavage of alkyl tert-alkyl ethers into the corresponding isoolefins and alkanols, comprising:
acid-catalyzed reactive distilling an alkyl tert-alkyl ether fed into the reaction zone of a reactive distillation apparatus as an azeotrope of the alkyl tert-alkyl ether and the corresponding alkanol, wherein the reactive distillation apparatus is configured from bottom to top as a bottom zone, at least one distillation zone and a reaction zone.
In another aspect of the invention, the alkyl tert-alkyl ether is fed into the reactive distillation apparatus below the reaction zone.